Liquid crystal display devices are used not only as small-sized display devices, e.g., the display sections of mobile phones, but also as large-sized television sets. Liquid crystal display devices of the TN (Twisted Nematic) mode, which have often been used conventionally, have relatively narrow viewing angles. In recent years, however, liquid crystal display devices with wide viewing angles have been produced, e.g., the IPS (In-Plane Switching) mode and the VA (Vertical Alignment) mode. Among such modes with wide viewing angles, the VA mode is adopted in a large number of liquid crystal display devices because of an ability to realize a high contrast ratio.
A liquid crystal display device includes an alignment film which defines an alignment direction of liquid crystal molecules in its neighborhood. In a VA-mode liquid crystal display device, an alignment film causes liquid crystal molecules to be aligned substantially perpendicularly to its principal face. Generic alignment films are made of polyimide, which has advantages in terms of thermal resistance, solvent resistance, hygroscopicity, and so on.
As one kind of VA mode, the MVA (Multi-domain Vertical Alignment) mode is known, under which a plurality of liquid crystal domains are created in one pixel region. An MVA-mode liquid crystal display device includes alignment regulating structures provided on the liquid-crystal-layer side of at least one of a pair of opposing substrates, between which a vertical-alignment type liquid crystal layer is interposed. The alignment regulating structures may be linear slits (apertures) or ribs (protruding structures) that are provided on electrodes, for example. The alignment regulating structures provide alignment regulating forces from one side or both sides of the liquid crystal layer, thus creating a plurality of liquid crystal domains (typically four liquid crystal domains) with different alignment directions, whereby the viewing angle characteristics are improved.
As another kind of VA mode, the CPA (Continuous Pinwheel Alignment) mode is also known. In a generic liquid crystal display device of the CPA mode, pixel electrodes of a highly symmetrical shape are provided, and on a counter electrode, protrusions are provided corresponding to the centers of liquid crystal domains. These protrusions are also referred to as rivets. When a voltage is applied, in accordance with an oblique electric field which is created with the counter electrode and a highly symmetrical pixel electrode, liquid crystal molecules take an inclined alignment of a radial shape. Moreover, the inclined alignment of the liquid crystal molecules are stabilized due to the alignment regulating forces of side slopes of the rivets. Thus, the liquid crystal molecules in one pixel are aligned in a radial shape, thereby improving the viewing angle characteristics.
Unlike in TN-mode liquid crystal display devices in which the pretilt direction of liquid crystal molecules is defined by an alignment film, alignment regulating forces in an MVA-mode liquid crystal display device are applied to the liquid crystal molecules by linear slits or ribs. Therefore, depending on distances from the slits and ribs, the alignment regulating forces for the liquid crystal molecules within a pixel region will differ, thus resulting in differing response speeds of the liquid crystal molecules within the pixel. Similarly, also in the CPA mode, the response speeds of the liquid crystal molecules will differ within the pixel, and the differences in response speed will become more outstanding as the pixel electrodes increase in size. Furthermore, in a VA-mode liquid crystal display device, the light transmittance in the regions in which slits, ribs, or rivets are provided is low, thus making it difficult to realize a high luminance.
In order to avoid the above problems, use of an alignment film for applying alignment regulating forces to liquid crystal molecules in a VA-mode liquid crystal display device is also known, such that the liquid crystal molecules will tilt from the normal direction of a principal face of the alignment film in the absence of an applied voltage (see, for example, Patent Documents 1 and 2).
In the liquid crystal display device disclosed in Patent Document 1, the alignment film is subjected to an alignment treatment such as rubbing. The alignment film aligns the liquid crystal molecules so that the liquid crystal molecules will be aligned with a tilt from the normal direction of its principal face even in the absence of an applied voltage, whereby an improved response speed is realized. Furthermore, since the alignment film defines the pretilt azimuth of liquid crystal molecules so that the liquid crystal molecules within one pixel will be symmetrically aligned, the viewing angle characteristics are improved. In a liquid crystal display device disclosed in Patent Document 1, four liquid crystal domains are formed in a liquid crystal layer in accordance with a combination of two alignment regions of a first alignment film and two alignment regions of a second alignment film, whereby a wide viewing angle is realized.
An alignment film disclosed in Patent Document 2 is made of a photosensitive material having a photoreactive functional group, and by obliquely radiating light onto this alignment film, a pretilt is conferred so that the liquid crystal molecules will be inclined from the normal direction of a principal face of the alignment film in the absence of an applied voltage. An alignment film to which a pretilt is conferred through such a photo-alignment treatment may also be referred to as a photo-alignment film. In the photo-alignment film disclosed in Patent Document 2, fluctuations in the pretilt angle are controlled to 1° or less, by using an alignment, film material which includes a bonded structure of photoreactive functional groups.
An alignment film which is made of one polymer may not attain adequate characteristics. Therefore, use of two different polymers to form an alignment film is under study (see Patent Document 3 and Non-Patent Document 1).
An alignment film disclosed in Patent Document 3 includes a main layer which is made of a first polymer having a large molecular weight and/or polarity, and a surface layer which is made of a second polymer having a small molecular weight and/or polarity. As the first polymer, a material including an aromatic (e.g., SE7690 manufactured by NISSAN CHEMICAL INDUSTRIES, LTD.), which hardly gives rise to any internal DC bias voltage, is used. On the other hand, the second polymer, which is a material that undergoes a large change in the pretilt angle in response to ultraviolet irradiation, is a cyclobutane-type polymer material. In Patent Document 3, SE7210 manufactured by NISSAN CHEMICAL INDUSTRIES, LTD. is used as the second polymer.
Non-Patent Document 1 discloses an alignment film including a lower layer whose main component is polyamic acid and an upper layer whose main component is polyimide. In Non-Patent Document 1, isolation into the two layers of the upper layer and the lower layer is achieved by appropriately setting a pre-bake temperature and time.